1. Field of the Invention
The present invention relates to a blow molding die which is useful in forming a blown film from a thermoplastic resin manifesting anisotropy in molten state, and a method for producing a blown film thereof.
2. Description of the Related Art
Among typical examples of thermoplastic resins manifesting anisotropy in molten state, so-called liquid crystal polymers are liquid crystal polyesters. This liquid crystal polyester is generally called a melting type liquid crystal (thermotropic liquid crystal) polymer and, does not cause entangling even in molten states owing to rigidity of the molecule, forms polydomain having liquid crystal property and reveals behavior that the molecular chain is significantly aligned to flow direction by low shear strength, unlike liquid crystal polymers such as polyethylene terephthalate and polybutylene terephthalate. The liquid crystal polyester has merits that it has excellent melt flow property, it can provide easily a thin wall molded article having about 0.2 to 0.5 mm thickness, and further this molded article shows high strength and high rigidity, owing to the above-described specific behavior.
On the other hand, a liquid crystal polyester is a polyester having feature that the molecule is oriented in molten state due to strong intermolecular mutual action, from the standpoint that it is a raw material of a film. Further, because of strong intermolecular mutual action and molecular orientation, the liquid crystal polyester has been expected to be used industrially as a film raw material having functions such as gas barrier property and the like in addition to well-known properties such as high strength, high elasticity and high heat resistance.
However, a liquid polyester has demerit that anisotropy is extremely high. Further, since a liquid crystal polyester does not cause entangling even in molten state and the molecule chain is significantly aligned to flow direction due to rigidity of the molecule as described above, it reveals behavior that melt viscosity decreases even by slight shear strength and melt tension in molten state is extremely low. Therefore, it is very difficult to keep form of a liquid crystal polyester in molten state, further properties of longitudinal and transverse directions are not easily balanced since the molecule is oriented, and in the extreme case, the liquid crystal polyester is torn along molecule orientation direction, leading to large problem that applicability in fields such as film forming, blow molding and the like is poor. Therefore, a liquid crystal polyester film utilizing functions of a liquid polyester has not been put into practical use to date.
Regarding such a liquid crystal polyester, trials have been made of a method for producing a film by blown film forming. In the blown film forming, a rein which has been melted and kneaded in an extruder is extruded to form cylindrical resin through an annular slit (resin flow route) of a cylindrical blow molding die, a certain amount of air is blown into the resulting cylindrical resin to expand the resin, and the expanded resin is continuously wound up with cooling to obtain a tubular film.
As a method for producing a film by such blown film forming, there is known for example that descried in Japanese Patent Application Publication (JP-B) No. 9-2635383. A blow molding die used in the method for producing a liquid crystal polyester film described in this publication has an inner lip, outer lip, inner mandrel and die body and further, an aeration hole for introducing air, and a polymer flow route utilizing a spiral mandrel for uniformalizing flow rate of polymer materials.
A liquid crystal polyester has properties that it has lower viscosity and flows easily as compared with polyethylene and polypropylene. Therefore, in the above-described conventional technologies, a part of a resin does not flow between spiral projections (spiral groove) formed on a spiral mandrel, but flows to upper direction passing through directly between the projections and the die body. Therefore, pressure dispersion effect by a spiral groove is not obtained, resin flow becomes ununiform along peripheral direction, as a result, weld line, unevenness in thickness and the like occur in the resulted film, accordingly, properties and appearance of the resulted film are not fully satisfactory.
On the other hand, among conventional blow molding methods are methods using a so-called spider die in which a mandrel is supported on a die body by a plurality of supporting means, other than a blow molding die comprising a spiral mandrel as described in the above-described conventional technology, so-called spiral die. However, this spider die has not been often used since flow controlling property is poor because a plurality of supporting means interrupt flow of a resin, resin flow along peripheral direction becomes ununiform as described above and therefore this die has not been believed to be suitable for a liquid crystal polyester having anisotropy.
The object of the present invention is to provide a blow molding die which makes it possible to obtain a film having excellent properties and good appearance in blown film forming of a thermoplastic resin manifesting anisotropy in molten state, and a method for producing such a film.
The present inventors have intensively studied, and as a result, found that a molten liquid crystal polyester can be allowed to flow through a resin flow route in a spider die with low pressure since this polyester has low viscosity and flows easily, and in this case, it is possible to support a mandrel on a die body in the view point of strength even if dimension of supporting means is made small or the number of supporting means is reduced.
Namely, the present invention provides a blow molding die used in forming a blown film from a thermoplastic resin which manifests anisotropy in molten state, wherein the die comprises a die body having a hollow portion, a mandrel placed in the hollow portion of the die body, a plurality of supporting means which are placed in an annular resin flow route formed in gap between the die body and the mandrel and support the mandrel on the die body and an aeration route for passing a gas to expand cylindrical resin extruded through a die gap (resin outlet part of the resin flow route), and the total maximum horizontal cross section of the plurality of supporting means in the resin flow route is one-fifth or less of the horizontal cross section of the resin flow route at any placing position of the plurality of supporting means.